Wire and cable insulations must meet both electrical properties and mechanical properties requirements. Long term exposure to high temperature cause deterioration in the physical properties of the insulation material. Polyethylene and polyvinylchloride compounds are some of the best materials for wire and cable insulations because of their excellent electrical and mechanical properties. However, these materials have major weaknesses due to their lack of flame retardancy and high generation of toxic gases.
Currently, fire resistance is obtained by using high level of mineral fillers (more than 140 phr). Halogenated resin such as polyvinylchloride (PVC) which have good fire resistance can also be used.
The excellent initial mechanical properties of thermoplastics such as ethylene vinyl acetate copolymers (EVA) or low density polyethylene (LDPE) are degraded when the level of metal hydroxides is higher than 100 phr.
Heat resistance is achieved by either chemical cross-linking, or radiation cross-linking. In both cases, chemically or radiation cross-linking, co-agent which is potential Volatile Organic Compound (VOC) is used. In the case of fire, the VOC combustion contributes to air pollution. There is a need for eco friendly products to reduce air pollution.
Li Chen et al. (2010) in their review describe several flame retardant technologies that have been used for the last 15 years. In this review, they discuss halogen free flame retardants extensively including mineral fillers, inorganic nitrogen, phosphorous substances and miscellaneous systems; the role of cross-linking is not discussed.
The effects of radiation cross-linking on the thermal degradation and flame retardant properties of HDPE/EVA magnesium hydroxide composites is described in Hui Liu et al. (2009). The cross-linking is provided via high energy electron beam irradiation at high absorbed dose up to 250 kGy. An co-agent, triallyl isocyanurate (TAIL), is used to promote-radiation cross linking.
Park et al. (US 2009/0020311 A1) describes a composition for a cable-covering material used in railway vehicles. A base resin including ethylene-vinyl acetate copolymers and a mixture of metal hydroxide as halogen free flame retardants are disclosed. The composition according to the reference needs a minimum of three parts in weight of a cross-linking agent in order to satisfy resistance against dynamic pressure at high temperature. The preferred way for the chemical cross-linking is by using peroxide.
Brown et al. (US200802511273 A1) indicates that excellent aging properties are reached by using VLDPE (very low density polyethylene) or ULDPE (ultra low density polyethylene) as base polymers instead of ethylene vinyl acetate copolymer (EVA). This invention relates to a plenum cable with excellent aging, electrical properties. No thermal properties are described.
Sato et al. (US2008015883 A1) indicates that flexibility, workability and compatibility with mineral filler are obtained by a blend of various styrene-ethylene block copolymers. Cross-linking is needed by irradiation at low dose. In order to complete cross-linking at low dose, some auxiliary co-agents are used. For the zinc compounds in this present invention zinc sulphide or zinc sulphate are used. Specifically preferred is zinc sulphide.
Luo et al. (U.S. Pat. No. 6,287,692) describes a composition of high density polyethylene (HDPE). As additive, an ethylene vinyl acetate copolymer (EVA) with high level in VA of more than 40% is introduced in the blends with the HDPE. Co-agent such as TMPTMA (trimethylol propane trimethacrylate) is used. The main goal of this invention is to meet fuel resistance requirements as the invention is particularly used in automotive gas tanks.
Fagher et al. (U.S. Pat. No. 6,043,312) concerns thermoplastic compositions with various fire retardants based on mixtures of metallic hydroxide and zinc borate. The resin blends are based exclusively on polyvinyl chloride (PVC).
And finally, Kwon et al. (U.S. Pat. No. 5,473,007) discloses some blends of Ethylene vinyl acetate copolymers (EVA) with chlorinated polyolefin's. The chlorinated polyolefin's have excellent flame retardancy due to chlorine level, but they generate toxic gases. Antimony trioxides are used at low level. Chlorinated polyolefin's associated with antimony trioxide provide a huge amount of toxic gases, during combustion, but LOI values reach up to 45%. Monomers with tri-functional groups at high level (from 5 to 20 phr) are needed to gain good mechanical properties. Phosphorous content in these compositions also pollutes air and soil. There is a need for an improved eco friendly wire and cable insulation material.